The ethanol that escapes FPM is distributed by the vascular system and diffuses to and from tissues across the capillary bed. As a small, highly polarized water-soluble molecule, ethanol diffuses into tissues in proportion to their water content (96). Ethanol does not bind to plasma proteins; in fact, ethanol can be used as a tracer to estimate total body water (TBW) with a precision that is on par with that obtained with the dilution method with heavy water (the gold standard method to determine TBW) (97, 98). The movement of ethanol is typically described as a passive diffusion process, where the intercompartmental concentration gradient is proportional to the rate of change. How fast alcohol equilibrates, even if briefly, between blood, extravascular fluids, and tissues depends on the cross-sectional area of the local capillary bed and the blood flow per gram of tissue (6). Therefore, highly perfused organs, with a higher blood flow rate per gram of tissue, such as the lungs, kidneys, brain, and liver, equilibrate more rapidly with arterial BAC than lower perfused ones, such as the skeletal muscles.
